Accept `void` as a parameter list

`foo(void)` indicate that function foo does not take a parameter.

parse.c

@@ -540,6 +540,11 @@ VarList *read_func_params() {
   if (consume(")"))
     return NULL;
 
+  Token *tok = token;
+  if (consume("void") && consume(")"))
+    return NULL;
+  token = tok;
+
   VarList *head = read_func_param();
   VarList *cur = head;
 
@@ -553,7 +558,7 @@ VarList *read_func_params() {
 }
 
 // function = type-specifier declarator "(" params? ")" ("{" stmt* "}" | ";")
-// params   = param ("," param)*
+// params   = param ("," param)* | "void"
 // param    = type-specifier declarator type-suffix
 Function *function() {
   locals = NULL;

tests

@@ -94,7 +94,7 @@ int count() {
 
 int param_decay(int x[]) { return x[0]; }
 
-void voidfn() {}
+void voidfn(void) {}
 
 int main() {
   assert(8, ({ int a=3; int z=5; a+z; }), "int a=3; int z=5; a+z;");